In recent years, multi-user multiple-input multiple-output (MU-MIMO) in which spatial multiplexing is performed on signals destined for a plurality of user terminal devices (user equipment, hereinafter referred to as “UE”) by using an identical radio resource has been attracting attention as a technology to increase capacity. In general, by calculating a transmission weight by the technique referred to as zero-forcing or minimum mean square error (MMSE) by using channel information between transmission antennas and the UEs to be multiplexed and multiplying transmission signals by the transmission weight, the interference among the UEs is reduced. That is, by multiplying the transmission signals by the transmission weight, beams for transmitting signals destined for the respective UEs are formed, and in the beam of the signal destined for a single UE, nulls for which the gain is small are directed to the directions of the other UEs.
However, depending on a combination of the UE to be multiplexed, even when the transmission signals are multiplied by the transmission weight, the nulls may be not directed to the UE mutually and the performance may deteriorate drastically. Accordingly, in MU-MIMO, it is desirable that the combination of the UE to be multiplexed be determined appropriately.
As for the method of determining a combination of the UEs to be multiplexed, there is a method that carries out transmission weight calculation and reception quality estimation on all possible combinations of the UEs and selects the combination for which a certain metric value becomes the largest. In such a method, when the number of UEs is large, the number of combinations of the UEs becomes enormous and a non-realistic computational complexity is needed. Thus, it has been conceived, by adding the UE one by one to the extent that the metric value is increased, to reduce the computational complexity in determining the combination of the UEs to be multiplexed.
Specifically, the reception quality γiMU of UE#i to be newly added is calculated by the following equation (1), for example.
                              γ          i          MU                =                                                            γ                i                SU                                                                                u                                                  +                1                                      ⁢                          (                              1                -                                                      ∑                                          k                      ∈                      u                                                        ⁢                                                                          ⁢                                      ρ                                          i                      ,                      k                                                                                  )                                =                                                    γ                i                SU                                                                                u                                                  +                1                                      ⁢                          (                              1                -                                                      ∑                                          k                      ∈                      u                                                        ⁢                                                                                                                                                                  h                            1                                                    ⁢                                                      h                            k                            H                                                                                                                      2                                                                                                                                                                      h                            i                                                                                                    2                                            ⁢                                                                                                                              h                            k                                                                                                    2                                                                                                        )                                                          (        1        )            
In equation (1), γiSU represents the reception quality of UE#i when the interference from the other UE is not present, and u represents a set of the UEs that have already been determined to be multiplexed when UE#i is added. hi is a channel response vector between UE#i and each transmission antenna, and when the channel response between UE#i and a transmission antenna #j is hi,j, hi is a matrix expressed like the following:hi=[h1,1h1,2 . . . hi,Ntx]
Ntx represents the number of transmission antennas. In equation (1), hH represents Hermitian transpose of matrix h, |⋅| represents an absolute value or a set size, and ∥⋅∥ is a symbol to represent a norm.
The metric value for which UE#i is added is calculated from the reception quality of the above-described equation (1), and when the metric value is increased by adding UE#i, UE#i is to be added to the combination of the UE to be multiplexed. Related art examples are described in Japanese Laid-open Patent Publication No. 2010-193189 and Japanese Laid-open Patent Publication No. 2009-278238, and in non-patent literatures of Jingxiu Liu, et al., A Low Complexity Capacity-Greedy User Selection Scheme for Zero-Forcing Beamforming, Vehicular Technology Conference, 2009. VTC Spring 2009, April 2009; of Takashi Seyama, et al., “Study of Coordinated Radio Resource Scheduling Algorithm for 5G Ultra High-Density Distributed Antenna Systems: Performance Evaluation of Joint Transmission Multi-User MIMO”, IEICE technical report, Vol. 115, No. 472, March 2016; and of Jun Sikida, et al., “Performance Evaluation of Low Complexity Multi-User MIMO Scheduling Schemes for Massive MIMO System”, IEICE technical report, Vol. 115, No. 472, March 2016.
However, there has been a problem in that, even when the UE is added one by one as in the foregoing, the computational complexity is not sufficiently reduced. That is, in the above-described equation (1), the processing of multiplying the channel response vector hi of the newly added UE#i by the channel response vector hk of each UE having already determined to be multiplexed is included. Assuming that UE#i is the m-th added UE and the number |u| of the UE having already determined to be multiplexed is (m−1), the real multiplication of 4·Ntx·(m−1) times is performed in determining the addition of UE#i. The number of times of the real multiplication increases as the number of antennas Ntx and the number of UEs to be multiplexed become large, and the growth of computational complexity can no longer be ignored. Consequently, as the result of the growth in computational complexity, the growth in the circuit scale also results.